Thermoplastic elastomers such as styrene-isoprene-styrene block copolymers (hereinafter abbreviated as SIS type block copolymers) and styrene-butadiene-styrene block copolymers (hereinafter abbreviated as SBS type block copolymers) have heretofore been used as base polymers for pressure sensitive adhesives and hot-melt type pressure sensitive adhesives. There has also been known a pressure sensitive adhesive composition making combined use of both SIS type block copolymer and SBS type block copolymer as a base polymer (Japanese Patent Publication No. 42345/1978).
However, both SIS type block copolymer and SBS type block copolymer involve a drawback that they are poor in heat stability (melt stability). The SIS type block copolymer undergoes heat deterioration upon its melt processing, and so its melt viscosity rapidly decreases. The SBS type block copolymer undergoes crosslinking upon its melt processing, and so its melt viscosity rapidly increases. With respect to blends of these block copolymers, the melt viscosity rapidly increases due to change with time upon its heating. Therefore, upon hot-melt processing, hot-melt type pressure sensitive adhesives separately comprising these block copolymers as base polymers are difficult to reliably keep stability to their quality upon coating operation and hot-melt processing. In order to stably keep performance characteristics such as pressure-sensitive adhesion properties, such hot-melt type pressure sensitive adhesives require to shorten the storage time of melts before coating and heating time upon processing as much as possible and to strictly control their melt processing temperature. Accordingly, these hot-melt type pressure sensitive adhesives are poor in operating ability upon coating.
On the other hand, in order to provide a pressure sensitive adhesive composition having high holding power, it has been proposed to use a branched block copolymer synthesized by using a trifunctional or still higher polyfunctional coupling agent, For example, International Publication No. 92/20725 discloses a process in which styrene is polymerized in the presence of an organic lithium initiator, isoprene is next added to polymerize it, a minor amount of butadiene is further added to form a short butadiene polymer block on a polymeric terminal of an isoprene polymer block, and these polymer blocks are then subjected to a coupling reaction with a polyfunctional coupling agent to produce a branched block copolymer. A pressure sensitive adhesive composition obtained by using, as a base polymer, a branched block copolymer synthesized by this process and having a high coupling rate exhibits relatively high holding power, but is low in initial adhesive strength and adhesive strength, and is hence not well balanced among pressure-sensitive adhesion properties.
The present inventors found that block copolymers of the SBIS type, (S--B--I).sub.n --X type and (S--I--B).sub.n --X type, which contain the copolymerizing components in specific proportions and have specific physical properties, have the characteristics as a thermoplastic elastomer and exhibit excellent heat stability (Japanese Patent Application Laid-Open No. 107745/1994). Here, n stands for an integer of 2-4, and X means a residue of a bi-, tri- or tetrafunctional coupling agent. A pressure sensitive adhesive making use of any one of these block copolymers as a base polymer scarcely undergoes a change of melt viscosity with time under high-temperature conditions and is well balanced among pressure-sensitive adhesion properties such as tackiness, adhesive strength and holding power. A pressure sensitive adhesive composition making use of the SBIS type block copolymer as a base polymer exhibits excellent heat stability when holding for about 15 minutes at 240.degree. C. However, its melt viscosity is decreased to a significant extent when holding for a long period of time at a high temperature (for example, for 72 hours at 180.degree. C.). A pressure sensitive adhesive composition making use of the (S--B--I).sub.n --X type or (S--I--B).sub.n --X type block copolymer involves a problem that its transparency may be impaired, or it may undergo discoloration when holding for a long period of time at a high temperature. In addition, it may be insufficient in heat resistance according to a heat resistance test (shear adhesive failure temperature test) in some cases. In particular, a pressure sensitive adhesive composition making use of the branched block copolymer, in which n is 3 or 4, as a base polymer is insufficient in tackiness and adhesive strength.
In order to improve the balance among the pressure-sensitive adhesion properties, it has heretofore been proposed to use, as a base polymer, a mixture of a branched block copolymer and a linear block copolymer (Japanese Patent Application Laid-Open No. 26938/1976), or to use a mixture of block copolymers, (A--B).sub.m --Y type and (A--B).sub.2 --X type block copolymers, synthesized by using a trifunctional or still higher polyfunctional coupling agent and a bifunctional coupling agent in combination (Japanese Patent Application Laid-Open No. 26647/1986). However, the pressure sensitive adhesive compositions making use of the conventionally-known mixtures of the block copolymers are not well balanced among pressure-sensitive adhesion properties such as tackiness, adhesive strength and holding power and are poor in heat stability or heat resistance. There is hence a demand for a further improvement.